Project 1 Summary T lymphocytes coordinate all aspects of adaptive immunity, including response to pathogens, the maintenance of immunological memory, and also direct many autoimmune and inflammatory diseases. Naive T cells emerge from the thymus, populate the blood and lymphoid tissues where they become activated by new antigens, differentiate into effector cells mediating antigen clearance, with a proportion surviving as long-lived memory T cells. It has been well-established in mouse models that memory T cells are heterogeneous in function and tissue distribution, while in humans T cells have largely been studied from peripheral blood. Recently, my laboratory and others identified a subset of tissue resident memory T cells (TRM) in mouse mucosal and barrier sites that are generated following infection, specifically retained in tissue sites of infection and mediate optimal protective responses to site-specific infections. TRM can also be generated to vaccines, tumors, allergens and autoantigens, indicating the potential importance of this key subset in immune protection and immunopathology. In the previous period of support, we have made novel use of a unique human tissue resource established as part of this Program to reveal how the major T cell subsets are distributed throughout the body at all stages of human life, generating an atlas of human T cell responses in space and time. Notably, we identified memory T cells as the predominant population in blood and multiple tissue sites, with TRM- phenotype cells comprising the majority of memory T cells in all tissues, including lymph nodes, spleen, bone marrow and mucosal sites. By whole transcriptome profiling by RNAseq, we identified a core signature for TRM in lymphoid and mucosal sites distinct from circulating TEM cells including expression of specific integrins, chemokine receptors, inhibitory molecules, and cytokines. These findings suggest a common precursor for TEM and TRM cells, likely derived from the initial activated and differentiated T cells. Our central hypothesis is that TRM and TEM derive from common precursors, but that TRM adopt tissue-specific differentiation and epigenetic profiles due to their localization and maintenance in specific sites. In the proposed research, we will dissect pathways for the generation and maintenance of tissue-resident memory T cells (TRM) relative to circulating effector-memory T cells (TEM) and assess whether TRM heterogeneity is based on function and/or localization. The proposed studies will elucidate mechanisms by which human TRM become localized in specific sites, and the effects of tissue niche in their signaling and function of great importance for optimizing tissue targeting of immune responses in vaccines and immunotherapies.